Available data suggest that if energy expenditure (EE) is involved in the etiology of obesity, the component of interest is activity-related EE (AEE). However, most prior studies examining the role of EE did not ensure metabolically stable conditions, include controls, or distinguish subjects according to ethnicity or predisposition to obesity. This study proposes to use a carefully controlled model of the post-obese state by reducing obese black and white women to a normal-weight post-obese state, and by comparing them with never-obese controls. 31P magnetic resonance spectroscopy is being used to access the ATP cost of force production associated with a controlled exercise task. Currently, we are in the 3rd year of acquiring data on this project and are in the final stages of compiling the first set of data from women that completed the NMR studies (30 Post-obese and 12 controls). In a preliminary analysis of the data, the post-obese subjects (n=12) show no differences in ATP production rates through the creatine kinase pathway, oxidative phosphorylation, or anaerobic glycolysis when compared with the matched controls (n=6). However, in this preliminary subset of subjects, the obese women show a reduced level of anaerobic glycolysis for energy production when compared to their post-obese state. These results need further investigation. Overall, this study has been designed to provide new insight into the potential contribution to obesity of variations in spontaneous AEE in adult black and white women, and into possible underlying mechanisms at the whole-body and biochemical levels. Such information is critical to understand the etiology of obesity and the potential importance of physical activity for its treatment and prevention, especially in the black population.